Graphene, a material comprising a lattice of carbon atoms positioned in a ‘honeycomb-type’ arrangement and tightly joined by in-plane sp2 bonds, has garnered much attention from research communities because its unique electrical and mechanical properties make it potentially ideally suited for various engineering applications, such as nanoelectronics and integrated circuits. Compared to other conductive and semiconductive materials, graphene has a superior carrier mobility and low resistivity, making it a promising candidate for integrated circuits. (Castro Neto A H, et al., Rev Mod Phys. 2009, 81, 109-62; Geim A and Novoselov K., Nature Materials. 2007, 6, 183-91; Novoselov K S, et al., Science. 2004, 306, 666-9; Novoselov K S, et al., Nature. 2005, 438, 197-200; Novoselov K S; McCann E, et al. Nat Phys. 2006, 2, 177-80; and Zhang Y, et al., Nature. 2005, 438, 201-4, the disclosures of which are incorporated herein by reference.)
However, graphene is inherently a semimetallic material—as opposed to a semiconductor material—and this limits its usability. (See, e.g., Oostinga J B, et al., Nature Materials. 2007, 7, 151-7; Ni Z H, et al., ACS nano. 2008, 2, 2301-5; Pereira V M, et al., Physical Review B. 2009, 80, 045401; Han M Y, et al., Phys Rev Lett. 2007, 98, 206805-8; Nakada K, et al., Phys Rev B. 1996, 54, 17954-61; and Ponomarenko L A, et al., Science 2008, 320, 356-8, the disclosures of which are incorporated herein by reference.) As a result, researchers have employed a number of methods to introduce a finite band gap within graphene, and thereby convert it into a semiconductor. One approach to introduce an energy gap opening in graphene is to break its lattice symmetry using foreign atoms such as hydrogen, gold, nitrogen, oxygen, and organic molecular dopants. (See, e.g., Bostwick A, et al., Physical Review Letters. 2009, 103, 056404; Balog R, et al., Nat Mater. 2010, 9, 315-9; Sessi P, et al., Nano letters. 2009, 9, 4343-7; Geirz I, et al., Nano Lett. 2008, 8, 4603-7; Wehling T, et al., Nano letters. 2008, 8, 173-7; Luo Z, et al., Appl Phys Lett. 2009, 94, 111909-11; Leconte N, et al., ACS nano. 2010, 4, 4033-8; Nourbakhsh A, et al., Nanotechnology. 2010, 21, 435203-11; Kim D C, et al., Nanotechnology. 2009, 20, 375703, Alzina F, et al., Physical Review B. 2010, 82, 075422 Gokus T, et al., ACS nano. 2009, 3, 3963-8; Childres I, et al., New Journal of Physics. 2011, 13, 025008; Dong X, et al., Small. 2009, 5, 1422-6; and Lu Y H, et al., The Journal of Physical Chemistry B. 2008, 113, 2-5, the disclosures of which are incorporated herein by reference.)
For example, researchers have used wet oxidation methods to insert foreign atoms into the graphene structure. These impurities alter the sp2 carbon hybridization in graphene to an sp3 carbon hybridization, and eliminate the π-bonds that facilitate charge transport across the graphene plane. Consequently, with diminished charge transport the desired band gap is obtained. (See, e.g., Elias D C, et al., Science. 2009, 323, 610-3; Sofo J O, et al., Physical Review B. 2007, 75, 153401; and Boukhvalov D W, et al., Physical Review B. 2008, 77, 035427, the disclosures of which are incorporated herein by reference.) However, the wet oxidation process is less than ideal: it typically uses harsh chemicals, such as strong acids and oxidizing agents; it takes a significantly long time to complete; and it does not allow for the creation of site specific oxidation, which substantially limits the usability of this modified graphene. (See, e.g., Hummers W S and Offeman R E, J Am Chem Soc. 1958, 80, 1339; Park S and Ruoff R, Nature nanotechnology. 2009, 4, 217-24; Li D, et al., Nat Nanotechnol. 2008, 3, 101-5; Sun X, et al., Nano Res. 2008, 1, 203-12; Becerril H A, et al., Nano Lett. 2008, 2, 463-70, the disclosures of which are incorporated herein by reference.)
Researchers have also experimented with using plasma oxidation, a dry oxidation method, to create a graphene oxide semiconductor material. This method is advantageous in a number of respects: it does not use any harmful chemicals; it is a more rapid process; and it allows for site specific oxidation. For example, Nourbakhsh et al. have fabricated and characterized such a graphene oxide layer. (See Bandgap Opening in Oxygen Plasma-Treated Graphene, Nourbakhsh et al. Nanotechnology 2010, 21, 435203-11, the disclosure of which is incorporated herein by reference.)
Previous studies also show that the p-doping level, electron-electron scattering rate, and the total density of states of an UV/ozone treated graphene are dictated by the defect density associated with surface concentration of oxygenated functional groups and oxygen molecule. At a very low defect density, the p-doping level and electron-electron scattering rate increase in proportion to the increase in defect density. At a higher defect density, a continuous decay and smoothing of the van Hove singularities becomes apparent, and a further increase in the defect density results in a significant drop in the conductance. This indicates a strong Anderson metal—insulator transition, with an overall change in the carrier concentration in the order of 1012 cm−2. These studies also show that an increase in defect density becomes increasingly difficult as the oxygen adsorption reaches a constant value after a certain UV/ozone exposure time. (See, e.g., Leconte N, et al., ACS nano. 2010, 4, 4033-8; Nourbakhsh A, et al., Nanotechnology. 2010, 21, 435203-11; Kim D C, et al., Nanotechnology. 2009, 20, 375703, Alzina F, et al., Physical Review B. 2010, disclosed above.)
Similar electronic transport behaviors are also observed in oxygen plasma treated graphene, where the p-doping level increases with the increase of oxygen plasma exposure, rendering the oxidized graphene unipolar. As the oxygen plasma exposure increases further, the level of disorder in the structural symmetry of graphene becomes more pronounced, which leads to a decrease in conductance and mobility, as well as a transition from semimetallic to semiconducting behavior. However, the prior art has yet to develop a process for the production of a semiconductor graphene oxide material suitable for practical applications. For example, although Nourbakhsh et al. discuss characterizing a graphene oxide layer created by a plasma oxidation process, the authors do not provide any guidance on how to avoid the creation of oxides on the substrate surface. For example, the authors incorrectly suggest that the band gap that can be created using this dry oxidation process can be as high as 3.6 eV (they reached this figure via calculation). It has now been discovered that such high band-gaps are impossible absent the destruction of the graphene oxide band gap. Accordingly a need exists for improved fabrication processes capable of forming graphene oxide materials in which the oxidation is confined within the graphene layer such that they can be used in practical applications.